The overall objective of the proposed work is the derivation and clinical utilization of more accurate electric and magnetic heart-lead relations than those presently known by accounting for the tissue electrical properties and configuration of the chest. The specific objectives are those of (1) developing a conductivity-corrected system of VCG leads by combining an improved triaxial lead system with precordial calibrations on each subject, (2) establishing by quantitative calculations the relations between depolarization sequences in the normal and abnormal heart and the complete surface potential maps, (3) deriving magnetic transfer coefficients to give the surface magnetic field arising from dipole sources at many points within the heart, (4) systematically evaluating the conditions under which a single dipole in a homogeneous conductor produces more than one each potential maximum and minimum on the body surface (5) developing theoretical models of the heart-lead relation that account for conductivity variations and torso shape; these will be simplified to permit intuitive inferences about source characteristics and put in perturbation form to permit evaluations of small deviations from an experimentally verified solution, (6) developing verified models of the heart-lead relation in the presence of extra-cardiac pathological conditions and (7) optimizing electric, magnetic and combined lead systems for external defibrillation and pacing. The methods to be employed utilize an anisotropic, inhomogeneous, twice- life-size model of the human torso as a source of electric and magnetic transfer coefficients, physical electromagnetic theory and patient studies. Data from the three approaches will be interrelated to obtain quantitative and verified results; the emphasis will be on investigations of whole body surface potential mass to maximize the amount of information available for interpretation.